We hope to better understand the ontogeny and phylogeny of immunity by exploiting unique amphibian models. With respect to ontogenesis, we shall explant small fragments of the dorsal blastopore lip of the gastrula stage and reciprocally transplant chromosomally labelled thymus anlagen of tail bud stage embryos in protocols that should allow us to: more precisely identify the original stem cell source of the early thymocyte; define early stem cell migration pathways; and evaluate (flow cytometry) the thymic origin of all B and T lymphocytes. We shall also employ immunofluorescent, mitogen, and MLR markers plus sequential thymectomy, embryonic anlagen grafting, and thymus reconstitution protocols to explore the interrelationships between the alternate pathway of lymphopoiesis and the peripheralization of thymocytes with the ontogeny of T-cell subsets. Our phylogenetic interest focuses on the evolution of T cells and MHC. We will determine whether salamanders and a primitive species of frog lack a T-cell subset found in advanced frogs. We will also test the concepts that the MHC evolved by convergent evolution, and that salamanders lack the complete MHC by carrying out immunogenetic analyses of MLR, GVH, and graft rejection in relevant species.